Heat exchanger with nests of tubes

ABSTRACT

Heat exchanger with nests of tubes having tube plates consisting of graphite. The tubes are cemented into recesses or holes of the tube plates. The depth of the recess is greater, preferably at least twice as great, as the height of the cement layer. The remaining ring gap is filled with substances in powder form or with such substances which do not bond with the cement mass.

United States Patent 1191 1111 3,782,450 Swozil Jan. 1, 1974 [54] HEAT EXCHANGER WITH NESTS ()F 2,956,787 10/1960 Raub 165/82 TUBES 3,701,548 10/1972 McGuire... 285/.016 3,182,720 5/1965 Brown 165/180 X [75] In nt Adolf Swozil, Ellgau Uber 1,992,504 2/1935 Penniman 165/81 Donauworth, Germany [73] Assignee: Sigri Elektrographit GmbH, FOREIGN PATENTS OR APPLICATIONS Augsburg, Germany 538,135 3/1957 Canada 285/D1G. 24

{22] Filed: Apr. 6, 1972 Primary Examiner-Charles .1. Myhre 21 A 1. N 241 585 1 pp 0 Assistant ExaminerTheoph1l W. Streule, Jr.

Attorney-Arthur E. Wilfond et al. [30] Foreign Application Priority Data Apr. 23, 1971 Germany P 21 19 820.9

57 ABSTRACT [52] US. Cl 165/81, 285/DIG. 16, 285/115, 1

285/189 165/180 Heat exchanger with nests of tubes having tube plates [51] 11 11. C] F281) 5/00 consisting of graphite The tubes are cemented into [58] Field Of Search 165/158, 175, 178, recesses or holesof the tube plates The depth of the 165/81, 82, 180; 285/189, 422, 114, 115, recess is greater, preferably at least twice as great, as the height of the cement layer. The remaining ring gap is filled with substances in powder form or with such [56] References C'ted substances which do not bond with the cement mass.

UNITED STATES PATENTS 3,327,777 6/1967 Kovalik et a1 165/180 X 3 Claims, 6 Drawing Figures PATENTEU 1 4 SHEEI 1 n; 3

FIG]

FIG. 2

PATENTEDJAN Hm 3,782,450

sum 2 or 3 Q FIG!- PATENTED 1 14 SHEET 3 OF 3 FIGS palm 2 HEAT EXCHANGER WTTH NESTS OF TUBES The present invention relates to a heat exhanger with nests of tubes; with graphite tube plates having recesses and graphite tubes engaging with the recesses. Annular cement joints are between the plates and the tubes.

For the elastic connection of the tubes of heat exchangers having nests of tubes with the tube plates, stuffing gland designs have become known which make possible an equalization of the expansion between the tubes and the enclosure of the apparatus. These designs, however, are expensive and cannot always be kept completely tight. According to another known design, the tubes are connected rigidly by welded, soldered or cemented joints with tube plates which are movable with respect to the enclosure and are sealed by means of diaphragm seals or stuffing glands. Temperature differences between the enclosure and the tubes cause relative motion of the plate, whereby the friction forces are transmitted by the tubes.

Heat exchangers with nests of tubes of graphite which exhibit very good corrosion resistance with high thermal conductivity, can be stressed with only relatively low forces because of the low tensile strength of graphite. In such exchangers, graphite tubes generally engage with cylindrical or conical recesses of a tube plate and the ring gaps between the tube and the holes are filled with a compound containing a hardenable synthetic resin. Hardening of the resin forms a firm and gas tight joint between the graphite tube and the tube plate. A drawback of such cement joints is that during the setting of the synthetic resin shrinkage stresses are generated which reduce the permissible stresses of the graphite tubes by tension or bending forces on the average of about 60 percent, so that even the relatively low strength of graphite cannot be utilized in designs.

It is an object of the present invention to avoid the strength reduction of graphite by the known cement joints and particularly to increase the allowable mechanical stresses of graphite tubes in heat exchangers with nests of tubes.

The problem is solved, according to the present invention, by having the depth of the recesses greater than the height of the cement joints and, preferably at least twice as great.

According to a preferred embodiment of the invention, the ring gap between the graphite tube and the cylindrical surface of the recess in the tube plate is filled with a cement compound which is rigidly connected only with the cylindrical surface or only with the graphite tube. A further embodiment of the invention provides the filling of the ring gap between the graphite tube and the cylindrical surface of the recess with an inert material in powder form.

For tube plates of less height, the invention provides ring washers which are cemented to the tube plate and are aligned with its recesses, the support surfaces of which have grooves, preferably.

According to a further preferred embodiment of the invention, the graphite tubes are led through support discs which are preferably arranged in at least two planes.

The invention is based on the recognition of the fact that in heat exchangers with nests of tubes according to the invention, graphite tubes which are deflected transversely to the longitudinal axis, are supported by support surfaces before the critical fracture stress is reached in the part of the tube weakened by the shrinkage stresses at the height of the annular cement joints. Bending moments, caused by tube vibrations or length changes, relative to the enclosure of the apparatus at the height of the cement joints, are limited. Maximum moments occur only in the unweakened tube parts whereby the probability of breakage of the tubes is decreased. The greater allowable mechanical stresses of heat exchangers with nests of tubes of graphite makes furthermore possible the use of graphite tubes with smaller wall thickness, thereby improving heat transfer.

The ratio of the depth of the recess to the height of the cement joint is determined by the maximally permissible deflection of the tubes and the tube tolerance. If the tubes are stressed in the flexural mode, they bear against the edges of the recesses and thereby limit the allowable stress of the cemented tube portions. If the tube tolerances are large or if the tube plates have only slight height, it is advantageous, according to the invention, to limit the lateral deflection of the tubes by filling the ring gap between the graphite tube and the cylindrical surface of the tube plate recess with cement or a material in powder form. Before applying the cement material, the graphite tubes or the cylindrical surfaces of the recesses must be coated with layers which do not form a firm bond with the cements used. Suitable substances for the preparation of these layers are, for instance, polytetrafluorethylene, lacquers, fats and silicon compounds. The generation of detrimental stress in the graphite tubes is avoided by the pretreatment, and the graphite tube is relieved of stress considerably at the height of the annular cement joint, even for a smaller ratio of the depth of the recess to the height of the cement.

In the case of low tube plates, which permit only very small free recesses, it is advantageous to arrange ring washers over the recesses and to reduce the clearance between the tube and the cylindrical surface of the washer, if desired, by a cement compound connected with the tube or with the cylindrical surface. The washers cemented to the tube plate absorb the stresses caused by the deflection of tubes, and cracks that occur in a washer do not propagate in the tube plate but are intercepted in the cement layer, the effect of which can be reinforced by grooves.

in heat exchangers with nests of tubes that have very thin tube plates, stresses at the height of the cement joints can be reduced, according to the invention, by means of support discs arranged above the tube plates in at least two planes. The distance between the tube plate and the support discs is obtained from the permissible deflection of the tubes and the tube tolerances. Laterally deflected tubes bear against the segment of ring fashioned discs and limit the mechanical stresses at the height of the cement zone.

The invention will be described with reference to embodiment Examples and the Drawings, in which:

FIG. 1 illustrates a known method of fastening of graphite tubes in a tube plate; and

FIGS. 2 6 illustrate heat exchangers with nests of tubes according to the invention.

FIG. 1 shows a prior art graphite tube plate with a bore 2 and a partially cylindrical and partially conical recess 3. In the recess 3, turned conically at the end, the graphite tube 4 engages and rests against the tube plate 1. The ring or annular gap 5 is fllled with a cement substance containing phenolic resin, graphite powder and an acidic catalyst, the setting of which results in a 3 rigid connection between the graphite tube and the tube plate. In the process, shrinkage stresses are set up in the cement mass which, in turn, causes a strength reducing stress in the graphite tube and the tube plate.

FlG. 2,.only the lower part of the ring gap 5 is filled with a cement substance whichafter setting, forms a firm cement joint. The free part; 6 of the ring gap is about 8 times longer than the height of the cement joint. in the event of a lateral deflection of the tube 4, the tube comes to rest against the edge 7, so that only a part of the bending stress is1 absorbed by the cemented, mechanically weakened part of the tube.

In FIG. 3, the tube plate 1 shows cylindrical bores or through holes 2 into which sleeves 8 are inserted from below for receiving the graphite tubes 4. As in the design of FIG. 2,'the maximum bending moments are shifted from the cement joints 9 to the edge 7.

For low or thin tube plates, which do not permit long free ring gaps, the design of FIGS. 4 and 5 are advantageous.

In FIG. 4, the conical part of the graphite tube 4 is rigidly connected with the tube bottom 1 by the cement joint 5. The cylindrical part of the recess is filled with the cement substance 10 and the outer cylindrical surface of the graphite tube has a thin layer of polytetrafluorethylene 11 by which a rigid bond of the cement with the graphite tube is prevented. This design permits only small transversal deflections even for short free ring gaps.

In the design according to FIG. 5, washers 12 are cemented, bycement 13, to the tube plate 1. The graphite tubes 4 have, at the height of the washers 12, a layer of silicon grease 11 by which a bonding of the tubes, with the cement substance, is prevented. Thus a rigid connection between the tube plate 1 and the grahite tube 4 exists only in the region of the conical recess. The washers 12 have grooves at their contact surfaces which prevent the propagation, into the tube plate, of cracks formed in the washers.

In FIG. 6, two support plates 15 are arranged in two planes above the tube plate 1. in the event of transversal deflections of a tube, it comes to rest against the support plate and relieves the cement joint 5. Passage of fluids outside of tubes 4 is shown by the arrows.

The advantages of the tube plates according to the invention are seen in the following Table:

T A B L E Known According to Instant lnvcntion Embodiment FIG. 1 FIG. 2 FIG. 4 FIG. 5 FTG. 6 maxfln 252 456 350 443 456 (kg/cm) m 'n 227 436 377 390 400 (kg/ m) 600 mm long graphite tubes with an outside diameter of 37 mm and an inside diameter of 25 mm were cemented into two plates withdiameters of 169 and 245 mm and thicknesses of and 140 mm, respectively, and stressed at a distance of 500 mm from the surface of the tube bottom until fracture occurred. The cement used contained 55 percent by weight of phenolic resin and 45 percent by weight of graphite powder. The setting time at t= was 6 hours.

In the above Table are given, besides the breaking stresses of the cemented-in tubes (0 the mean breaking strength of free graphite tubes (07, tube). The ratio o' /o' tube gives the usable strength of the respective design relative to the tube strength. in the averate, the permissible strength of graphite tubes in heat exchangers with nests of tubes according to the invention is about twice as great as in heat exchangers known up to now.

I claim:

1. A heat exchanger comprising a plurality of graphite tubes, a graphite plate arranged transversely of the tubes and having recesses each of which receive an end of a respective one of the tubes, the relative diameters of the ends of the tubes and the recesses being such that an annular space is defined between the end of each of the tubes and the recess in which the tube is received, a cement substance filling each annular space and cementing the tube into the recess over only a portion of the axial dimension of the annular space beginning at the free end of the tube, the remainder of the axial dimension of the annular space being free over at least a portion of its radial dimension.

2. A heat exchanger according to claim 1, in which the portion of the annular space which is filled by the cement substance extends from the free end of the tube a distance no greater than one-half the total axial dimension of the annular space.

3. A heat exchanger according to claim 1, in which the remainder of the axial dimension of the annular space contains the cement substance over only a portion of its radial dimension beginning at and adhering to the wall of the tube or the wall of the recess defining the annular space. 

1. A heat exchanger comprising a plurality of graphite tubes, a graphite plate arranged transversely of the tubes and having recesses each of which receive an end of a respective one of the tubes, the relative diameters of the ends of the tubes and the recesses being such that an annular space is defined between the end of each of the tubes and the recess in which the tube is received, a cement substance filling each annular space and cementing the tube into the recess over only a portion of the axial dimension of the annular space beginning at the free end of the tube, the remainder of the axial dimension of the annular space being free over at least a portion of its radial dimension.
 2. A heat exchanger according to claim 1, in which the portion of the annular space which is filled by the cement substance extends from the free end of the tube a distance no greater than one-half the total axial dimension of the annular space.
 3. A heat exchanger according to claim 1, in which the remainder of the axial dimension of the annular space contains the cement substance over only a portion of its radial dimension beginning at and adhering to the wall of the tube or the wall of the recess defining the annular space. 